Metal sulfide filled carbon nanotubes and synthesis methods thereof

ABSTRACT

Filled carbon nanotubes (CNTs) and methods of synthesizing the same are provided. An in situ chemical vapor deposition technique can be used to synthesize CNTs filled with metal sulfide nanowires. The CNTs can be completely and continuously filled with the metal sulfide fillers up to several micrometers in length. The filled CNTs can be easily collected from the substrates used for synthesis using a simple ultrasonication method.

GOVERNMENT SUPPORT

This invention was made with government support under DMR1506640 awardedby the National Science Foundation. The government has certain rights inthe invention.

BACKGROUND

Carbon nanotubes (CNTs) are the allotropes of carbon that are formed byrolling up an individual or several graphitic layers in a concentricfashion. The conversion of a two-dimensional graphene layer into aone-dimensional tubular structure having a diameter in a nanoscale andlength of several micrometers gives rise to new physical properties inCNTs. For example, an individual CNT can either have characteristicsthat are metallic or semiconducting depending upon its diameter andhelicity, which makes its electronic properties different from that ofsemi-metallic graphene/graphite. Since their discovery, CNTs have drawnmuch attention because of their unique electronic, mechanical, andthermal properties making them attractive for several physicalapplications.

Besides the fundamental physical properties of CNTs, their unique hollowtubular structure allows the storage of foreign materials in the CNTcore to bring about a novel class of hybrid nanomaterials. These hybridnanomaterials (or nanohybrids) can be thought as nano-capsules ornano-cocoons formed with one or several cylindrical carbon shellsencapsulating a foreign material usually in form of nanoparticles (NPs),nanorods, or nanowires. The insertion of guest materials in the hollownanochannels of CNTs can integrate the properties of host CNTs and guestfillers simultaneously, thereby resulting in distinctive physicalproperties that are different from that of either the empty CNTs or thefilling materials. The synergistic effects of carbon shells and theguest fillers distinguish the filled CNTs as novel nanohybrids withunusual electrical, magnetic, electromagnetic, electrochemical, andoptical properties. These new properties of filled CNTs can expand theirapplications in various devices such as lithium-ion batteries (LIBs),microwave absorbers, biosensors/chemosensors, data storage units, drugdelivery systems, nanothermometers, nanofilters, and memory devices. Thefilling process also serves as a route towards the generation ofdifferent kinds of nanowires (e.g., metallic, semiconducting, magnetic)that have inspired much interest because of their low dimensions,enlarged surface-to-volume ratio, and quantum confinement effects.

BRIEF SUMMARY

Embodiments of the subject invention provide novel and advantageousfilled carbon nanotubes (CNTs) and methods of synthesizing the same. Insitu methods (e.g., chemical vapor deposition techniques) can be used tosynthesize CNTs (e.g., multi-walled CNTs) filled with metal sulfide(e.g., nickel sulfide, iron sulfide, cobalt sulfide) nanowires. The CNTscan be completely (or nearly completely) and continuously (or nearlycontinuously) filled with the metal sulfide fillers (e.g., metal sulfidenanowires, such as single crystalline nanowires) up to severalmicrometers (or several tens of micrometers) in length. The filled CNTscan be easily collected from the substrate(s) used for synthesis using asimple ultrasonication method, and the same substrate(s) can be reusedto synthesize filled CNTs again.

In an embodiment, a method of synthesizing filled CNTs can comprise:providing a substrate in a reaction chamber; disposing a catalystcomprising a metal on the substrate; providing a sulfur-containingorganic compound to the reaction chamber; and performing a chemicalvapor deposition technique to simultaneously grow CNTs (e.g.,multi-walled CNTs) and fill the CNTs with a metal sulfide comprising themetal, thereby synthesizing the filled CNTs filled with the metalsulfide. The CNTs can have an average filling ratio (the proportion ofthe CNT that is filled as a fraction of the total CNT) of the metalsulfide of at least 0.90, or even at least 0.99. The metal can be, forexample, nickel, cobalt, or iron (e.g., the metal can be nickel and themetal sulfide can be Ni₃S₂). The substrate can be, for example, siliconor a foam (e.g., a foam comprising the metal). The sulfur-containingorganic compound can be, for example, thiophene or dimethyl sulfide. Theproviding of the sulfur-containing organic compound to the reactionchamber can comprise: providing a supply of the sulfur-containingorganic compound outside the reaction chamber; and sending a gas throughthe supply of the sulfur-containing organic compound and into thereaction chamber such that the sulfur-containing organic compound isbubbled into the reaction chamber with the gas. The method can furthercomprise collecting the filled CNTs via ultrasonication, and may furthercomprise using a magnet to purify the filled CNTs during the collecting,after the collecting, or both. The reaction chamber can comprise, forexample, a tube of a first material (e.g., quartz) with a boat (e.g., ofthe first material) disposed therein, and the boat can comprise asurface to which the substrate is provided. The method can also furthercomprise, after collecting the filled CNTs, reusing the substrate tosynthesize further filled CNTs (e.g., by repeating the synthesis stepsdescribed herein). In addition, the method can further comprise, beforedisposing the catalyst on the substrate, cleaning the substrate viaultrasonication using at least one solvent (e.g., by performing threesuccessive ultrasonication processes using deionized water, acetone, andisopropyl alcohol, respectively).

In a further embodiment, the catalyst can be a salt of the metal, andthe method can further comprise: before providing the sulfur-containingorganic compound to the reaction chamber, heating the reaction chamberto a first predetermined temperature (e.g., 600° C.) for a firstpredetermined amount of time (e.g., 30 minutes) to oxidize the catalyst;after oxidizing the catalyst, flowing an inert gas through the reactionchamber and passing hydrogen gas through the reaction chamber to reducethe catalyst; and after reducing the catalyst, stopping the flowing ofthe hydrogen gas through the reaction chamber while continuing to flowthe inert gas through the reaction chamber while heating the reactionchamber to a second predetermined temperature (e.g., in a range of 1000°C.-1200° C.). The disposing of the catalyst on the substrate cancomprise: preparing a catalyst solution by mixing a powder of the saltof the metal in a solvent; providing the catalyst solution to thesubstrate; and performing a two-stage spin coating to form a uniformcoating of the catalyst on a surface of the substrate. The catalystsolution can have a concentration of the salt of the metal in a range of0.02 molar (M) to 0.06 M.

In a further embodiment, the catalyst can be nanoparticles of the metal,and the method can further comprise: before disposing the catalyst onthe substrate, dispersing the nanoparticles in a solvent and putting thesolvent in an ultrasonication bath to achieve a uniform suspension ofthe nanoparticles in the solvent; dip coating at least a portion of thesubstrate by immersing the at least a portion of the substrate in acontainer having the suspension of the nanoparticles in the solvent andgently shaking the container; and transferring the at least a portion ofthe substrate to a heating apparatus (e.g., a hot plate) and heating (ata temperature of, e.g., 150° C. for, e.g., 5 minutes) the at least aportion of the substrate to evaporate the solvent. The substrate can bea foam comprising the metal (e.g., nickel foam).

In another embodiment, a compound can comprise: a CNT (e.g., amulti-walled CNT); and a nanowire of a metal sulfide filled in the CNT.The CNT can have a filling ratio of the metal sulfide of at least 0.90(or even 0.99), and the CNT can have a length of at least 5 μm. The CNTfilled with the nanowire can be straight from one end (i.e., a first endthereof) to the other end (i.e., a second end thereof). Each of thefirst end and the second end of the multi-walled CNT filled with thenanowire can be closed with carbon shells. The metal sulfide can be, forexample, nickel sulfide, cobalt sulfide, or iron sulfide (e.g., themetal sulfide can be Ni₃S₂, Co₉S₈, or Fe_(x)S_(y)).

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of a setup for in situ synthesis of filledcarbon nanotubes (CNTs) using a chemical vapor deposition technique,according to an embodiment of the subject invention.

FIG. 2 a is a scanning electron microscope (SEM) image of 0.06 molar (M)catalyst particles after dispersion. The scale bar is 100 μm.

FIG. 2 b is an SEM image of the 0.06 M catalyst particles of FIG. 2 a ,after oxidation. The scale bar is 100 μm.

FIG. 2 c is an SEM image of the 0.06 M catalyst particles of FIG. 2 a ,after reduction. The scale bar is 100 μm.

FIG. 2 d is an SEM image of a single catalyst particle after dispersion.The scale bar is 10 μm.

FIG. 2 e is an SEM image of a single catalyst particle after oxidation.The scale bar is 10 μm.

FIG. 2 f is an SEM image of a single catalyst particle after reduction.The scale bar is 10 μm.

FIG. 2 g is a high magnification SEM image of a single catalyst particleafter dispersion. The scale bar is 2 μm.

FIG. 2 h is a high magnification SEM image of a single catalyst particleafter oxidation. The scale bar is 2 μm.

FIG. 2 i is a high magnification SEM image of a single catalyst particleafter reduction. The scale bar is 2 μm.

FIG. 3 a is an SEM image of Ni₃S₂ filled CNTs synthesized usingNi(NO₃)₂.6H₂O IPA suspensions of a concentration of 0.02 M. The scalebar is 20 μm.

FIG. 3 b is an SEM image of Ni₃S₂ filled CNTs synthesized usingNi(NO₃)₂.6H₂O IPA suspensions of a concentration of 0.04 M. The scalebar is 20 μm.

FIG. 3 c is an SEM image of Ni₃S₂ filled CNTs synthesized usingNi(NO₃)₂.6H₂O IPA suspensions of a concentration of 0.06 M. The scalebar is 20 μm.

FIG. 3 d is a high magnification SEM image of Ni₃S₂ filled CNTssynthesized using Ni(NO₃)₂.6H₂O IPA suspensions of a concentration of0.02 M. The scale bar is 10 μm.

FIG. 3 e is a high magnification SEM image of Ni₃S₂ filled CNTssynthesized using Ni(NO₃)₂.6H₂O IPA suspensions of a concentration of0.04 M. The scale bar is 10 μm.

FIG. 3 f is a high magnification SEM image of Ni₃S₂ filled CNTssynthesized using Ni(NO₃)₂.6H₂O IPA suspensions of a concentration of0.06 M. The scale bar is 10 μm.

FIG. 4 a is a chart of the distribution of length (in μm) of Ni₃S₂filled CNTs synthesized using catalyst at a concentration of 0.02 M.

FIG. 4 b is a chart of the distribution of diameter (in nm) of Ni₃S₂filled CNTs synthesized using catalyst at a concentration of 0.02 M.

FIG. 4 c is a chart of the distribution of length (in μm) of Ni₃S₂filled CNTs synthesized using catalyst at a concentration of 0.04 M.

FIG. 4 d is a chart of the distribution of diameter (in nm) of Ni₃S₂filled CNTs synthesized using catalyst at a concentration of 0.04 M.

FIG. 4 e is a chart of the distribution of length (in μm) of Ni₃S₂filled CNTs synthesized using catalyst at a concentration of 0.06 M.

FIG. 4 f is a chart of the distribution of diameter (in nm) of Ni₃S₂filled CNTs synthesized using catalyst at a concentration of 0.06 M.

FIG. 5 a is an SEM image of Ni₃S₂ filled CNTs synthesized using catalystat a concentration of 0.01 M. The scale bar is 5 μm.

FIG. 5 b is an SEM image of Ni₃S₂ filled CNTs synthesized using catalystat a concentration of 0.07 M. The scale bar is 5 μm.

FIG. 6 a is an SEM image of pristine nickel foam (Ni—F). The scale baris 100 μm.

FIG. 6 b is an SEM image of catalyst loaded Ni—F. The scale bar is 100μm. The inset, with a scale bar of 200 nanometers (nm), shows nickelnanoparticles deposited on the surface of the Ni—F.

FIG. 6 c is an SEM image of Ni₃S₂ filled CNTs grown on the Ni—F. Thescale bar is 100 μm.

FIG. 6 d is an SEM image showing the straight structure of the Ni₃S₂filled CNTs with closed tips. The scale bar is 10 μm.

FIG. 6 e is a high magnification SEM image showing a clear filling ofCNTs with Ni₃S₂ nanowires and the spherical roots of the Ni₃S₂ filledCNTs. The scale bar is 1 μm.

FIG. 6 f is an SEM image showing Ni₃S₂ filled CNTs collected viaultrasonication and deposited on a piece of silicon wafer. The scale baris 20 μm.

FIG. 7 a is an SEM image showing Ni₃S₂ filled CNTs after collection viaultrasonication followed by a magnetic purification method. The scalebar is 5 μm.

FIG. 7 b is an SEM image showing Ni₃S₂ filled CNTs after collection viaultrasonication followed by a magnetic purification method. The scalebar is 2 μm.

FIG. 8 a is an SEM image of Ni—F after collection of Ni₃S₂ filled CNTscollected via an ultrasonication process. The scale bar is 100 μm.

FIG. 8 b is a higher magnification SEM image of the Ni—F aftercollection of Ni₃S₂ filled CNTs collected via an ultrasonicationprocess. The scale bar is 20 μm.

FIG. 8 c is an SEM image of the Ni—F substrate of FIGS. 8 a and 8 bafter regrowth of Ni₃S₂ filled CNTs on that same substrate. The scalebar is 100 μm.

FIG. 8 d is a higher magnification SEM image of the Ni—F substrate ofFIGS. 8 a and 8 b after regrowth of Ni₃S₂ filled CNTs on that samesubstrate. The scale bar is 20 μm.

FIG. 9 a is a transmission electron microscope (TEM) image showingcomplete and continuous filling of CNTs. The scale bar is 5 μm.

FIG. 9 b is a TEM image showing an individual Ni₃S₂ filled CNT having aspherical root. The scale bar is 1 μm. The inset, with a scale bar of200 nm, shows the closed tip structure.

FIG. 9 c is a high resolution TEM image showing an individual Ni₃S₂filled CNT. The scale bar is 5 nm. The insets show the selected areaelectron diffraction (SAED) pattern and lattice fringes.

FIG. 9 d is a drift-correct scanning transmission electron microscope(STEM) image of an individual Ni₃S₂ filled CNT. The scale bar is 1 μm.The inset, with a scale bar of 200 nm, shows the high-angle annulardark-field imaging (HAADF)-STEM image.

FIG. 9 e is an energy dispersive X-ray (EDX) image of elemental mappingof C—K; the scale bar is 200 nm.

FIG. 9 f is an EDX image of elemental mapping of Ni—K; the scale bar is200 nm.

FIG. 9 g is an EDX image of elemental mapping of Ni—L; the scale bar is200 nm.

FIG. 9 h is an EDX image of elemental mapping of S—K; the scale bar is200 nm.

FIG. 9 i is the spectrum (counts versus energy (in kilo-electron Volts(KeV))) for C—K, Ni—K, Ni-L, and S—K.

FIG. 10 a is an X-ray diffraction (XRD) spectrum of Ni₃S₂ filled CNTs.

FIG. 10 b is Raman spectra of empty CNTs (bottom curve withI_(D)/I_(G)=1.06) and Ni₃S₂ filled CNT (upper curve withI_(D)/I_(G)=0.26). The inset shows the Raman spectra of Ni₃S₂ filledCNTs at lower wavenumbers.

FIG. 11 a is the UV-Vis a plot of the spectra of differentconcentrations of empty CNTs.

FIG. 11 b is the UV-Vis a plot of the spectra of differentconcentrations of Ni₃S₂ filled CNTs.

FIG. 12 is a plot of absorption versus concentration (in milligrams perliter (mg/l)) for empty CNTs at ultraviolet 276 nm (the higher line withcircular data points) and Ni₃ S₂ filled CNTs (the lower line with squaredata points).

FIG. 13 a is an SEM image of cobalt sulfide nanowires filled in CNTs(Co₉S₈ filled CNTs) synthesized using cobalt nanoparticles on a siliconsubstrate. The scale bar is 10 μm.

FIG. 13 b is an SEM image of Co₉S₈ filled CNTs synthesized using cobaltnanoparticles on a silicon substrate. The scale bar is 10 μm.

FIG. 13 c is an SEM image of Co₉S₈ filled CNTs synthesized using cobaltnanoparticles on a silicon substrate. The scale bar is 1 μm.

FIG. 13 d is an SEM image of Co₉S₈ filled CNTs synthesized using cobaltnanoparticles on a silicon substrate. The scale bar is 1 μm.

FIG. 14 a is an SEM image of iron sulfide nanowires filled in CNTs(Fe_(x)S_(y) filled CNTs) synthesized using iron nanoparticles on a Ni—Fsubstrate. The scale bar is 10 μm.

FIG. 14 b is an SEM image of Fe_(x)S_(y) filled CNTs synthesized usingiron nanoparticles on a Ni—F substrate. The scale bar is 10 μm.

FIG. 14 c is an SEM image of Fe_(x)S_(y) filled CNTs synthesized usingiron nanoparticles on a Ni—F substrate. The scale bar is 10 μm.

FIG. 14 d is an SEM image of Fe_(x)S_(y) filled CNTs synthesized usingiron nanoparticles on a Ni—F substrate. The scale bar is 1 μm.

FIG. 15 is a schematic view of a setup for in situ synthesis of filledsingle-walled carbon nanotubes (SWCNTs) using a floating catalystchemical vapor deposition technique, according to an embodiment of thesubject invention.

DETAILED DESCRIPTION

Embodiments of the subject invention provide novel and advantageousfilled carbon nanotubes (CNTs) and methods of synthesizing the same. Insitu methods (e.g., chemical vapor deposition techniques) can be used tosynthesize CNTs (e.g., multi-walled CNTs or single-walled CNTs) filledwith metal sulfide (e.g., nickel sulfide, iron sulfide, cobalt sulfide)nanowires. The CNTs can be completely (or nearly completely) andcontinuously (or nearly continuously) filled with the metal sulfidefillers (e.g., metal sulfide nanowires, such as single crystallinenanowires) up to several micrometers (or several tens of micrometers) inlength. The filled CNTs can be easily collected from the substrate(s)used for synthesis using a simple ultrasonication method, and the samesubstrate(s) can be reused to synthesize filled CNTs again.

Sulfur-containing compounds, such as sulfur-containing hydrocarboncompounds (e.g., thiophene or dimethyl sulfide) can be used tosynthesize filled CNTs (e.g., metal sulfide filled CNTs). Metalnanoparticles (e.g., nickel nanoparticles, cobalt nanoparticles, or ironnanoparticles) or metal salts (e.g., metal nitrate (such as nickelnitrate, iron nitrate, or cobalt nitrate), metal sulfate, or metalchlorate) can be used as the catalyst material for synthesis of metalfilled CNTs. A spin coating technique can be used to prepare catalystprecursors on a substrate, and/or a dip coating technique can be used toload metal nanoparticles on a metal foam substrate. Ultrasonication canbe used to collect filled CNTs after synthesis, and a magnet can be usedto purify the filled CNTs during/after collection. Any suitablesubstrate (e.g., silicon or a metal foam such as nickel foam) can beused in the synthesis. Substrates can be reused/recycled to synthesizefilled CNTs again after collection of a previous batch of synthesizedfilled CNTs. A metal foam substrate, if used, can be pretreated toincrease efficiency of synthesis. The concentration of catalystprecursor can be varied/chosen to control the length and/or diameter ofsynthesized filled CNTs. Also, different concentrations of metalnanoparticle-solvent suspension can be used to control the amount ofmetal sulfide nanowire filled CNTs synthesized in a metal foamsubstrate.

Embodiments of the subject invention provide compounds that comprise aCNT (e.g., a multi-walled CNT) and a nanowire of a metal sulfide filledin the CNT. The CNT can have a filling ratio of the metal sulfide of atleast 0.90 (or even 0.99), and the CNT can have a length of at least 5μm. The CNT filled with the nanowire can be straight from one end (i.e.,a first end thereof) to the other end (i.e., a second end thereof). Eachof the first end and the second end of the CNT filled with the nanowirecan be closed with carbon shells. The metal sulfide can be, for example,nickel sulfide, cobalt sulfide, or iron sulfide (e.g., the metal sulfidecan be Ni₃S₂, Co₉S₈, or Fe_(x)S_(y)).

The structure of the filler has been confirmed by X-ray diffraction(XRD) measurements, and the quality of the filled CNTs was assessed byRaman spectroscopy; a significantly low value of the ratio between theintensities of D and G bands (e.g., I_(D)/I_(G)=0.26) show that the CNTshells of the filled CNTs (e.g., Ni₃ S₂ filled CNTs) are exceptionallywell-graphitized and of ultra-high quality. Electron microscopymeasurements have also confirmed that CNTs can be completely andcontinuously filled with single crystalline nanowires of Ni₃S₂(heazlewoodite) (e.g., rhombohedral Ni₃S₂) up to several micrometers (orseveral tens of micrometers) in length. Ni₃S₂ filled CNTs exhibit uniqueabsorption properties in the UV-Vis region due to the transfer ofelectrons from carbon shells to the Ni₃S₂ nanowire. Any suitablesubstrate can be used, including but not limited to conductivethree-dimensional nickel foam (3D Ni—F) or silicon. Ni₃S₂ filled CNTssynthesized on conductive 3D Ni—F substrates (Ni₃S₂@CNTs/Ni—F) have manypotential applications, including but not limited to anode materials forhigh capacity lithium-ion batteries and electrocatalysts for watersplitting reactions.

Filled carbon nanotubes (CNTs) exhibit unique physical propertiesarising from the synergistic effects of the carbon shells and thefilling material, making them attractive for numerous applications.Although inorganic and organic materials have been successfullyencapsulated inside CNTs, there is still a lack of a reliable,efficient, scalable, and economical method in the related art to achievea complete (or even near complete) filling of CNT cores with transitionmetal sulfide nanowires.

In general, there are two different approaches of filling CNTs that areknown as ex situ filling and in situ filling. Ex situ filling is amulti-step filling of the cores of carbon nanotubes by foreign elements,which can also be referred to as post-synthesis filling. There are threemajor steps involved in this filling process: (1) synthesis of carbonnanotubes; (2) opening the closed caps of carbon nanotubes; and (3)filling of the open carbon nanotubes. Opening of carbon nanotubes isusually realized by oxidation using oxygen (O₂) and acid. Once the endsof carbon nanotubes are opened, they can be filled by foreign materialsin several ways. Most common filling techniques are a physical method(e.g., capillary induced filling) and wet chemical method. The physicalmethod uses an annealing at a temperature that is higher than themelting point of a filling precursor. The precursor deposited in thesurface of carbon nanotubes can melt and eventually impregnate insidecarbon nanotubes whose tips remain open during annealing. The wetchemical technique uses an acidic treatment to open the tips of carbonnanotubes and a solution of precursor that can fill carbon nanotubesduring oxidation and transform chemically after furtherheating/reduction. The latter approach is widely used because it favorsthe filling with a heterogeneity of elements and the formation of singlecrystalline metallic nanostructures inside carbon nanotubes.

The ex situ approach is limited by following factors. (1) The opening ofcarbon nanotubes is a destructive process, and it damages the walls ofthe carbon nanotubes while it opens the carbon nanotube tips. (2) Theopening efficiency is highly dependent on the degree of dispersion ofthe carbon nanotubes. (3) The fillers are often segments or particlesresulting in non-complete (or non-uniform) filling of the carbonnanotubes. (4) The fillers inside the open carbon nanotubes are notprotected at the ends.

In situ filling of carbon nanotubes refers to a single-step fillingprocess. Unlike ex situ filling, which requires the opening of nanotubeends, this one-step synthesis is able to generate carbon nanotubes withends closed at both sides, keeping the capsule intact and the corematerial preserved from undesired chemical reactions (e.g., oxidation)with the surrounding environment. Also, this process is applicable toelements that have high surface tension and do not apply withcapillarity or wetting techniques. Being a single step method, thisprocess is a quicker way of filling carbon nanotubes, ensuring lessexertion and an ease of control during the filling process over severalparameters that are responsible for the results. Commonly used methodsfor in situ filling of carbon nanotubes include the arc discharge methodand chemical vapor deposition (CVD). These methods can successfully fillcarbon nanotubes with several metals, alloys, and metallic compounds.Though, the arc discharge method suffers serious limitations such asdifficulty in filling carbon nanotubes by pure metals (as they usuallyreact with carbon to form metal carbides), lower efficiency,infeasibility to fill carbon nanotubes by multi-component fillers suchas metal alloys, and poor control over the filling process likely due tohigh temperature gradients that are characteristic of a plasma reactor.

CVD is a synthesis process that involves a catalytic activity of metalor metallic compounds involved in the system. It facilitatessimultaneous growth of carbon nanotubes and metal, which catalyzes thegrowth so as to encapsulate the excess of catalyst inside carbonnanotubes usually in form of nanowires. Unlike arc discharge, CVD worksat relatively lower temperatures typically using metal catalystdeposited in the form of a thin film or powder on substrates. Metalsulfide (e.g., nickel sulfide, iron sulfide, and cobalt sulfide) filledcarbon nanotubes have not been successfully synthesized using the insitu method in the related art. Although an alloy of iron sulfide andnickel sulfide has been filled inside carbon nanotubes (see Su et al.,In Situ Synthesis of Iron/Nickel Sulfide Nanostructures-Filled CarbonNanotubes and Their Electromagnetic and Microwave-Absorbing Properties,The Journal of Physical Chemistry C 2011, 115 (5), 1838-1842, which ishereby incorporated by reference herein in its entirety), the filler wasnot a pure single phase iron sulfide or nickel sulfide but an alloy ofthem because stainless steel was used as the metal precursor thatcontains mainly iron and nickel elements, the filling rate was very lowmeaning only a small fraction of the carbon nanotubes were filled, andall the filled carbon nanotubes were curved. Among the filled carbonnanotubes, many of them were partially filled. Cobalt sulfide (Co₉S₈)has also been filled inside carbon nanotubes (see Du et al., FillingCarbon Nanotubes with Co9S8 Nanowires through in Situ CatalystTransition and Extrusion, J. Phys. Chem. C, 112 (2008) 1890-1895, whichis hereby incorporated by reference herein in its entirety), a mixtureof cobalt salt (Co(NO₃)₂.6H2O and magnesium salt (Mg(NO₃)₂.6H₂O) wasused as a catalyst precursor, and magnesium oxide (MgO) resulted fromthe magnesium salt decomposition was present as an impurity in thesynthesized materials and was difficult to remove, and the filling rateof the carbon nanotubes was low. Embodiments of the subject inventionare able to synthesize metals sulfide (including nickel sulfide, ironsulfide, and cobalt sulfide) filled carbon nanotubes with a very highfilling rate and high purity. The core of the carbon nanotube iscompletely (or nearly completely) filled with the metal sulfidenanowires, and metal sulfide nanowires filled carbon nanotubes arealways (or at least predominantly) straight (and not curved).

As discussed above, ex situ filling is a multi-step filling process thatis also referred to as post synthesis filling because it requires thesynthesis of CNTs as the first step, opening of CNT tips as the secondstep, and filling of CNTs as the final step. On the other hand, in situfilling is a single step filling process in which CNTs are synthesizedand filled simultaneously such that the final result is a filled CNThaving its ends closed by carbon shells keeping the CNT intact and thuspreserving the filler from any chemical reaction with the surroundingenvironment. In situ filling is a more advanced technique that allowsthe complete (or near complete) filling of CNT cores with a continuousnanowire of the filler whereas the ex situ method typically results in apartial filling, the filler remaining as nanoparticles, nanocrystals, ornanorods. Another big advantage of in situ filling is the quality of theproducts as this method does not require any form of acid treatment orannealing to open the CNT tips, which would cause severe damage to theCNT walls inducing structural defects. Moreover, it is also applicabletowards filling CNTs with various elements that bear a high surfacetension (e.g., greater than 200 milliNewtons per meter (mNm⁻¹)) andcannot be sucked into the CNT core by capillary action due to theviolation of wetting conditions. Also, being a quick one-step process,it is easy to control the growth and filling parameters in the in situmethod, which makes it more reliable and efficient.

CNTs have been filled with a wide variety of both inorganic and organicmaterials, but related art filled CNTs are typically achieved using theex situ method. An efficient in situ filling of CNTs with long andcontinuous nanowires of a material has remained a challenge in therelated art. There exists a need in the art for in situ filling of CNTswith heterogeneity of materials, and the commercialization of such hasremained elusive due to the dearth of the amount that can be synthesizedusing the existing techniques. In other words, a small quantity ofX@CNTs (where “X” is the filling material) typically synthesized on atwo-dimensional substrate such as silicon or stainless steel can onlyserve as a primary model to understand some basic properties of thenanohybrid but it is unfeasible to explore their properties in moredetail. Also, many application tests demand a large quantity of X@CNTsalong with a high quality, which entails an up-scaled and reliablesynthesis technique.

An attractive material is the nanostructured nickel (Ni) sulfides due totheir extraordinary physical properties. However, their encapsulationinside CNTs has not been achieved previously. The in situ synthesis ofnickel sulfide filled CNTs is viable because transition metals such asNi have a few d-vacancies and act as some of the best catalysts tonucleate and grow CNTs. The presence of sulfur (S) during the materialsynthesis also induces the graphitization of carbonaceous materials.Besides, a sulfidation reaction can proceed when a proper amount of Satoms is released on the nickel catalyst nanoparticles (NPs) forming anickel sulfide nucleus, which not only promotes the CNT growth but alsogets extruded into the CNT in form of a long nanowire.

Nickel sulfide can exist in various stable phases with simplest to morecomplex stoichiometric forms such as α-NiS, β-NiS, NiS₂, Ni₃S₂, Ni₃S₄,Ni₇S₆, Ni₉S₈, Ni₁₇S₁₈, etc.. Among these various members of the nickelsulfide family, Ni₃S₂ (heazlewoodite) is attractive due to its intrinsicmetallic behavior, rich abundance, high stability, high energy capacity,environmentally friendly behavior, and remarkable electrocatalyticproperties. The physical properties of Ni₃S₂ can be further elevated byincorporating it with carbon materials. For example, Ni₃S₂ nanocrystalswrapped with a few layers of carbon and anchored on graphene sheets(Ni₃S₂@C/RGO) can exhibit discharge capacities up to 850 and 630 mAhg⁻¹at current densities of 200 and 5000 mAg⁻¹, respectively, when evaluatedas lithium ion battery (LIB) anodes (see Guan et al., Carbon WrappedNi₃S₂ Nanocrystals Anchored on Graphene Sheets as Anode Materials forLithium-Ion Battery and the Study on Their Capacity Evolution,Nanomaterials (Basel), 8 (2018) 760, which is hereby incorporated byreference herein in its entirety). The enhancement in theelectrochemical performance of Ni₃S₂ is attributed to the wrappingeffect by carbon (C) layers, which can prevent the structurepulverization of Ni₃S₂ nanocrystals during Li⁺ insertion/desertion.Similarly, carbon coated Ni₃S₂ nanowires behave as an efficientelectrocatalyst for hydrogen evolution reactions exhibiting a potentialof about 199 mV at 10 mAcm⁻² along with a prolonged stability over 30hours at a current density of 20 mAcm⁻² (see Li et al., One-stepsynthesis of Ni₃S₂ nanowires at low temperature as efficientelectrocatalyst for hydrogen evolution reaction, International Journalof Hydrogen Energy, 42 (2017) 7136-7142, which is hereby incorporated byreference herein in its entirety). Likewise, graphene decorated Ni₃ S₂(Ni₃ 5 ₂@G) pyramids synthesized on three-dimensional nickel foam (3DNi—F) show improved electrocatalytic activity for both hydrogenevolution reactions and oxygen evolution reactions during watersplitting experiments (see Yu et al., In-situ growth of graphenedecorated Ni₃ S₂ pyramids on Ni foam for high-performance overall watersplitting, Applied Surface Science, 465 (2019) 772-779, which is herebyincorporated by reference herein in its entirety). The enhancement isdue to the synergistic effects of Ni₃S₂ and graphene, as well as thebinder free nature of the Ni₃S₂@G electrode firmly attached onthree-dimensional (3D) nickel foam (Ni—F) substrates having a largesurface area and a high porosity. Ni₃S₂@CNTs exhibit unique physicalproperties and can find numerous applications in the fabrication of nextgeneration electronic or electrochemical devices.

Embodiments of the subject invention include CNTs (e.g., multi-walledCNTs or single-walled CNTs) filled with metal sulfide. The filled CNTscan have an average filling ratio (the proportion of the CNT that isfilled to the total CNT) of 0.80 or higher and an average length of 5 μmor higher. The average filling ratio can be any of the following values,about any of the following values, at least any of the following values,or greater than any of the following values (all values are unitless):0.80, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, 0.90, 0.91,0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 0.995, or 1.0. Theaverage length of the CNTs can be any of the following values, about anyof the following values, at least any of the following values, orgreater than any of the following values (all values are in μm): 4, 5,6, 7, 8, 9, 10, 10.5, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 20.16, 21,22, 23, 24, 25, 30, 34.93, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85,90, 95, or 100. As seen in FIGS. 3 a-3 f, 6 c-6 f, 7 a, 7 b, 8 c, 8 d, 9a-9 d, 13 a -13 d, and 14 a-14 d, the filling ratio of the CNTs preparedin the examples is very high (>0.98, and about 1.0 in some cases) andthe lengths are long. These extremely high filling ratios have not beenobtained previously in metal sulfide filled CNTs, and it is due to theefficient in situ method using CVD as disclosed herein. The embodimentsof the subject invention have also overcome the problem in the relatedart where oxides (e.g., MgO) could be difficult to eliminate from filledCNTs such that the filled CNTs with oxide present had low purity.

Embodiments of the subject include in situ methods to synthesizemulti-walled or single-walled CNTs filled with metal sulfide (e.g.,nickel sulfide) nanowires. Different precursors and/or substrates can beused. For example, Ni₃S₂@CNTs can be grown on silicon (Si) substrateswith a control over their lengths and diameters, and/or a significantmass of Ni₃S₂@CNTs can be synthesized on conductive 3D Ni—F substrates.Both of these techniques lead to the growth of high-quality CNTscompletely and continuously filled with Ni₃S₂ nanowires severalmicrometers (e.g., at least 5 μm) in length. These synthesis methods canalso be implemented to fill CNTs with other transition metal sulfidesincluding but not necessarily limited to iron sulfide and cobaltsulfide.

In many embodiments, metal (e.g., nickel, iron, cobalt) nanoparticles ora salt thereof (e.g., metal nitrate (such as nickel nitrate, ironnitrate, or cobalt nitrate), metal sulfate, or metal chlorate) and asulfur-containing compound (e.g., thiophene or dimethyl sulfide) can beused as the precursors for the synthesis of metal sulfide filled CNTs atlow cost. The CNT growth and filling of metal sulfide occurssimultaneously (in situ filling as opposed to ex situ filling thatinvolves opening CNTs first and then filling them). Unlike related artCNTs that are sporadically filled with filler particles or segments, theCNTs in embodiments of the subject invention are uniformly filled withlong continuous metal sulfide nanowires. The technology disclosed hereinenables largescale production of metal sulfide filled CNTs.

Embodiments of the subject invention provide synthesis methods of metalsulfide filled CNTs using metal nanoparticles or their salts and asulfur-containing compound (e.g., a sulfur-containing hydrocarbon suchas thiophene or dimethyl sulfide) as precursors to in situ synthesizethe metal sulfide nanowire filled CNTs via a CVD method. This in situgrowth process includes the CNTs and metal sulfide nanowires inside theCNTs being grown simultaneously in a one-step and easily controlledprocess. The metal sulfide nanowires are continuous crystallinenanowires encapsulated inside closed carbon nanotubes. This is the mostreliable and efficient way of producing closed carbon nanotube hybridsin situ. It is an extremely useful technique to fill carbon nanotubeswith long continuous nanowires of metal (e.g., Fe, Co, Ni) sulfides withease and at low cost. The filled CNTs have applications in, for example,material science, electronic devices, energy, and environmentalprotection.

Specific applications include but are not limited to lithium ionbatteries, microwave absorbers, sensing probes for magnetic forcemicroscopy, filtration of toxic elements from fluids, magnetic recordingand data storage devices, biological/chemical sensors, drugdelivery/carrier systems, and nanothermometers. As an example, metalsulfides are promising anode materials for energy storage for lithiumion batteries as they have much higher lithium ion storage capacity thanthe currently commercially used graphite electrodes. However, metalsulfides tend to break due to their large volume change during thecharge-discharge cycles, so the breakdown of the metal sulfides cancause the lithium ion battery to fail. The encapsulation of the metalsulfides by the CNTs inhibits or prevents the metal sulfide breakdownbecause the strong CNT protects the metal sulfide from collapse. Inaddition, the excellent electrical conductivity of the CNTs improves theelectron conduction during the charge-discharge cycles. As a result,lithium ion batteries using metal sulfide filled CNTs as anode materialshave significantly improved performance.

Filled CNTs of embodiments of the subject invention have manyapplications. The solid core-shell structure with highly ordered carbonshells, better electronic conductivity, and mechanical strength (and insome cases direct growth of filled CNTs on conductive Ni—F) make suchfilled CNTs excellent candidates for various applications such as inenergy storage devices (lithium ion batteries (LIBs) andsupercapacitors), electrocatalysis of water, and nanoelectromechanicalsystems (NEMS). For example, one major application of Ni₃S₂@CNTs is touse them in energy storage devices such as LIBs and supercapacitors.Given the high theoretical lithium storage capacity (445 mAhg⁻¹) ofNi₃S₂, mechanical strength offered by CNT walls, better electronicconductivity of both CNTs and Ni₃S₂, and a high specific surface area ofthe 3D Ni—F substrate, Ni₃S₂@CNTs (e.g., Ni₃S₂@CNTs grown on nickel foamsubstrates (Ni₃S₂@CNTs/Ni—F)) electrodes can dramatically improve theelectrochemical performance of LIBs and supercapacitors. Theincorporation of nanostructured Ni₃S₂ with several carbonaceousmaterials is promising as LIB anodes. Ni₃S₂@CNTs provide excellentresults due to the complete encapsulation of the filler by carbonshells, which can easily inhibit or prevent the structural degradationof Ni₃S₂ during lithiation and de-lithiation cycles. Moreover, thecontinuity of Ni₃S₂ nanowire between the CNT tips and roots providesmore active sites for electrochemical reactions and also facilitates afast transfer of electrons and ions. Another important application ofNi₃S₂@CNTs can be envisioned in water splitting reactions. Severalnanostructures of Ni₃S₂ and their composites have garnered intenseconcerns in hydrogen evolution reaction (HER), oxygen evolution reaction(OER), and oxygen reduction reaction (ORR) due to their outstandingelectrocatalytic activities. However, pristine Ni₃S₂ catalysts losetheir efficiency and stability over time due to their structurepulverization caused during oxidation/reduction cycles. Theencapsulation of Ni₃S₂ by robust CNT walls can overcome this problem,and Ni₃S₂@CNTs can act as efficient electrocatalysts for HER, OER, andORR. Meanwhile, the binder free nature of Ni₃S₂@CNTs/Ni—F electrodeensures high activity and stability. Also, Ni—F itself increases thecatalytic activity due to its high specific surface area and alsoenhances the ionic/electronic conductivity due to its open porestructure. In addition to these major applications, Ni₃S₂@CNTs may findother interesting applications in nanoelectrochemical systems (NEMS)because of the mechanical reinforcement possessed by the solidcore-shell structure and good electronic conductivity of both the CNTsand Ni₃S₂.

A greater understanding of the embodiments of the subject invention andof their many advantages may be had from the following examples, givenby way of illustration. The following examples are illustrative of someof the methods, applications, embodiments, and variants of the presentinvention. They are, of course, not to be considered as limiting theinvention. Numerous changes and modifications can be made with respectto the invention.

Materials and Methods

Nickel nitrate hexahydrate (Ni (NO₃)₂.6H₂O, 99% pure) was obtained fromFisher Scientific International, Inc. Ni nanoparticles (NPs) (99.9%, 40nm, metal basis) were obtained from US Research Nanomaterials, Inc., andNi foam (99.99%, 1.6 mm thick, 80-100 pores per inch) was obtained fromMTI Corporation. All reagents—isopropyl alcohol (IPA), acetone, andethanol—were obtained from Fisher Scientific and were used as received.Thiophene (C₄H₄S) of extra pure grade (99+%) was obtained from AcrosOrganics and was used as the precursor for both the carbon and sulfur.Empty multi-walled CNTs (used as reference) were synthesized using amethod similar to that previously reported (see Xie et al., Large-ScaleSynthesis of Aligned Carbon Nanotubes, Science, 274 (1996) 1701 and Panet al., Very long carbon nanotubes, Nature, 394 (1998) 631-632; both ofwhich are hereby incorporate by reference herein in their entireties).Briefly, CNTs were synthesized on iron/silica substrates by thepyrolysis of acetylene. The iron/silica substrates were formed by asol-gel process in which tetraethoxysilane and iron nitrate aqueoussolution were used as the precursors and concentrated hydrogen fluoridewas used as additive to control the gelation speed. After gelation, thegel was dried overnight at about 100° C. to form substrates, and thenthe substrates were calcined at 450° C. overnight. For the carbonnanotube synthesis, the substrates were reduced at 550° C. for 5 hoursin a flow of 9% H₂/N₂ with a flow rate of 110 cm³/min to yield ironnanoparticles on the surface of silica. Subsequently, carbon nanotubeswere produced on the substrates in a flow of 9% acetylene in nitrogenwith 110 cm³/min flow rate at 650° C. for 24 hours. The setup shown inFIG. 1 was used for synthesis of filled CNTs.

Scanning electron microscope (SEM) images for characterization wereacquired by using Field Emission Scanning Electron Microscope (FESEM)JEOL JSM-6330F with an accelerating voltage of 25 kilovolts (kV).Transmission electron microscopy (TEM) images, energy dispersive X-ray(EDX) spectra, and selected area electron diffraction (SAED) patternsfor characterization were collected by using a Tecnai G² 20 U-Twinhigh-resolution transmission electron microscope with an accelerationvoltage of 200 kV. X-ray spectroscopy for characterization was performedon powder samples using Siemens Diffraktometer D5000 with Cu Kαradiation (λ=1.54Å). Raman spectroscopy for characterization wasperformed using a Raman microscope setup described with a laserwavelength of 632.8 nm (see Guo et al., Monitoring the Dynamic Processof Formation of Plasmonic Molecular Junctions during Single NanoparticleCollisions, Small, 14 (2018) 1704164; which is hereby incorporatedherein by reference in its entirety). UV-Vis spectra forcharacterization were collected in an Agilent Technologies Cary SeriesUV-Vis-NIR spectrophotometer.

EXAMPLE 1

Synthesis of Ni₃S₂@CNTs using nickel salt

Three different catalyst solutions with concentrations of 0.02molar (M),0.04 M and 0.06 M, respectively, were prepared by mixing crushed powderof Ni (NO₃)₂.6H₂O and IPA. 5 microliters (μl) of the catalyst solutionwas dropped on the surface of a silicon (Si) substrate cleaned byultrasonication using de-ionized (DI) water, acetone, and IPAsuccessively. A thin film of catalyst layer was then formed on thesurface of the Si substrate using a two-stage spin coating. The spinrates were optimized as 850 rpm for 60 seconds in the first stage and2000 rpm for another 60 seconds in the second stage to form a uniformcoating of catalyst particles on the surface of the substrate.

Three to four substrates coated with a catalyst layer were placed in aquartz boat and aligned to the center of a quartz tube, which was thenheated in a furnace at 600° C. for 30 minutes to oxidize the catalystlayer. Then, argon (Ar) was passed through the tube via valve A (seeFIG. 1 ) at a flow rate of 100-200 standard cubic centimeters per minute(sccm) and H₂ at 100-200 sccm for 15 minutes, which were responsible forcreating an inert atmosphere and reducing the catalyst particles,respectively. Valve C was closed during the reduction step such that H₂gas could pass through valve B. After reduction, the flow of H₂ wasstopped whereas Ar continued to flow until the temperature of thefurnace reached 1000° C.-1200° C. The flow rate of Ar was then increasedto 1650-1850 sccm and H₂ was resumed to flow at 100-200 sccm but along anew flow path via valve C. Such flow could bubble the H₂ throughthiophene (C₄H₄S), which was used as a precursor for both carbon andsulfur. During a synthesis time of 10-30 mins, CNTs could grow and fillthemselves with Ni₃S₂ nanowires simultaneously. After the reaction timewas completed, the flow of H₂ was cut off and the reaction chamber wascooled down under an inert atmosphere keeping Ar to flow continuously at100-200 sccm.

The images of a representative single particle after dispersion,oxidation, and reduction are shown in FIGS. 2 d-2 f , respectively.Referring to 2 d, a smooth dome-shaped surface can be seen on the microhemisphere surrounded by a ring-like structure, which then ruptures intoa rose-like surface with multiple layers after oxidation in air at 600°C. as shown in FIG. 2 e . FIG. 2 f represents the micro hemisphere afterreduction by H₂ gas showing a further fissure of the surface formingseveral porous clusters of particles. A more detailed structure of thesurface of the hemispherical particle is shown in the high magnificationimages of FIGS. 2 g-2 i . The original catalyst particle (shown in FIG.2 g ) gets thermally decomposed in several steps and finally convertsinto nickel oxide (NiO) at a temperature of about 600° C., and the hightemperature causes multiple fractures of the particle surface as shownin FIG. 2 h . Upon reduction by H₂ gas, NiO converts into metallic Niand the prolonged exposure in high temperature leads to the furthermodification of the surface as shown in FIG. 2 i . The multiple layersobserved in FIG. 2 h disappeared after reduction, and the surface thenincluded an abundance of NPs of metallic Ni as seen in FIG. 2 i . Thebig clusters can further break down into smaller Ni nanoparticles as thetemperature increases during the synthesis process. These Ni NPs are theactive sites for nucleation that can form Ni₃ S₂ through a sulfidationreaction and catalyze the growth of CNTs simultaneously.

The SEM images of three different samples synthesized via nickel saltusing the catalysts of three different concentrations 0.02 M, 0.04 M,and 0.06M are shown in FIGS. 3 a-3 f . These results correspond to thesynthesis conditions in which the flow rate of H₂ and Ar were 90 sccmand 1550 sccm, respectively, during a growth time of 15 minutes at 950°C. Both the lower magnification SEM images in FIGS. 3 a-3 c and thehigher magnification SEM images in FIGS. 3 d-3 f demonstrate successfulgrowth of Ni₃S₂@CNTs for each concentration of the catalyst. Arelatively higher yield, along with bigger dimensions of Ni₃S₂@CNTs, wasobserved for the catalyst concentration of 0.06 M.

It can be seen in FIGS. 3 a-3 f that the CNTs produced by this methodare straight and tapered with spherical roots and very sharp tips. Incontrast, empty CNTs are usually curved and entangled due to radialbuckling. Thus, the linear structure of Ni₃S₂@CNTs suggests that theyare completely filled with Ni₃S₂ nanowires between their roots and tips(i.e., filling ratio of the nanowire in the CNT of at least 0.99 (e.g.,1.0)).

The effect of the concentration of catalyst precursor on the dimensionsand morphology of Ni₃S₂@CNTs was investigated by measuring the lengthsand diameters of 100 different Ni₃S₂@CNTs from various samplessynthesized by using each of the three different concentrations of thecatalyst precursor. It was found that the higher concentration ofcatalyst led to the synthesis of longer and thicker Ni₃S₂@CNTs; however,the morphology of Ni₃S₂@CNTs remained unchanged. Corresponding to thecatalyst concentrations of 0.02M, 0.04M and 0.06M, the lengths ofNi₃S₂@CNTs were mainly distributed in the range of 5-15 μm, 15-25 μm,and 30-40 μm, respectively, whereas the diameters measured at theirmiddle section were distributed in the range of 50-150 nm, 100-200 nm,and 200-300 nm, respectively. These results are shown in FIGS. 4 a -4 f.

It was found that catalyst precursor concentrations of 0.01 M and 0.07 Mled to a lower yield of Ni₃S₂@CNTs as shown in FIGS. 5 a and 5 b ,establishing the preferred range for the catalyst concentration for thegrowth of Ni₃S₂@CNTs as 0.2 M-0.06 M.

EXAMPLE 2

Synthesis of Ni₃S₂@CNTs using Nickel Nanoparticles

The synthesis method using nickel salt (Example 1) is useful to controlthe lengths and diameters of Ni₃S₂@CNTs, but the growth scale islimited. Thus, another method was developed to synthesize a largequantity of Ni₃S₂@CNTs on nickel foam (Ni—F) substrates.

Ni nanoparticles (NPs) were dispersed in IPA (20-100 grams per liter(g/l)) and kept in an ultrasonication bath for 5 minutes to achieve auniform suspension of nanoparticles, and then the suspension wastransferred in a vial ready for use. Ni—F was cut into small pieces ofroughly 2 cm×2 cm in size, and the pieces were cleaned in DI water,acetone, and IPA successively by ultrasonication. The Ni—F substrateswere individually immersed in a vial containing the IPA-Ni NPssuspension and left for dip coating for 5 minutes. During that time, thevial was gently shaken to prevent or inhibit the agglomeration of Ni NPsat the bottom of the vial. Then, the Ni—F was transferred to a hot platemaintained at 150° C. and heated for 5 minutes to evaporate the IPA fromthe substrate. The CVD growth of Ni₃S₂@CNTs was carried out on the Ni—Fsubstrates under the same conditions as described in Example 1, but itis required that Ni—F should undergo a pre-treatment in similarexperimental conditions before loading the catalyst particles. The firststep (oxidation in air) was eliminated because the catalysts were puremetal NPs. The synthesis methods described in Examples 1 and 2 aresummarized in Table 1.

TABLE 1 CVD growth parameters for Examples 1 and 2. ExperimentalConditions Synthesis Method Precursor/Substrate Oxidation ReductionSynthesis Via Ni salt Ni (NO₃)₂•6H₂O/Si 600° C. Ar: 80-200 sccm Ar:1500-1900 sccm (Example 1) 30 mins H₂: 80-200 sccm H₂: 80-200 sccm 10-20minutes 10-30 minutes T = 500-650° C. T = 850-1200° C. Via Ni NPs NiNPs/3D Ni-F* N/A Ar: 80-200 sccm Ar: 1500-1900 sccm (Example 2) H₂:80-200 sccm H₂: 80-200 sccm 10-20 minutes 10-30 minutes T = 500-650° C.T = 850-1200° C. *Substrate pre-treatment required under identicalexperimental conditions before catalyst loading

The SEM images of Ni₃S₂@CNTs synthesized using Ni NPs are depicted inFIGS. 6 a-6 f . Pristine nickel foam (Ni—F) loaded with nickel catalystnanoparticles cannot promote the growth of Ni₃S₂@CNTs. Thus, apre-treatment of Ni—F substrates under similar experimental conditionsis necessary to activate the growth of Ni₃S₂@CNTs from Ni NPs depositedon Ni—F. The main reason behind this situation can be a change in therelative surface energies between Ni—F and Ni NPs as the Ni—F surfaceundergoes a sulfidation reaction along with carbon diffusion during thepre-treatment process. The pristine Ni—F (FIG. 6 a ) gets uniformlycovered with Ni NPs as shown in FIG. 6 b after a pre-treatment processfollowed by dip-coating with Ni NPs IPA suspension. The 3D structure ofNi—F can accommodate a big mass of Ni NPs, which are the active sitesfor the growth of Ni₃S₂@CNTs. Fortunately, the catalyst loading can becontrolled by changing the suspension concentrations. For instance, aparticular Ni—F substrate of 2 cm×2 cm, as shown in FIG. 6 c , dipcoated with a suspension having a concentration of 80 g/l, couldaccommodate 10.19 mg of Ni NPs after completely drying off.

Referring to FIGS. 6 c-6 e , almost all Ni catalyst particlesparticipated in the growth of Ni₃S₂@CNTs. Also, the Ni—F itself (withoutloading of catalyst particles) undergoes an increase in mass by 6.3%after pre-treatment under identical synthesis conditions as a result ofsulfidation reaction and carbon diffusion that can occur on the Ni—Fsurface. Thus, the mass of Ni₃S₂@CNTs can be estimated as the differencebetween the final mass of the Ni—F substrate containing Ni₃S₂@CNTs andthe sum of the initial mass of Ni—F before loading the catalystparticles and its increased mass during nanotube synthesis process. Thisgives the mass of Ni₃S₂@CNTs as 4.10 mg in the aforementioned sample.Because a typical experiment can use 3-4 Ni—F substrates of 2 cm×2 cmsize, about 5-20 mg of Ni₃S₂@CNTs can be synthesized in a singleexperiment. However, with an increased size of Ni—F substrate along withan increased mass of catalyst loading, this method can be applicable tosynthesize Ni₃S₂@CNTs at an industrious scale.

FIG. 6 c plainly shows that Ni₃S₂@CNTs can grow in several layers ofNi—F, confirming that the multi-layered structure of a substrate such asNi—F can be advantageous for growing a large quantity of Ni₃S₂@CNTs.Also, a high density of Ni₃S₂@CNTs observed on the lateral surface(edges) of the Ni—F layers as shown in FIG. 6 d suggests that the porousstructure of Ni—F also facilitates the growth of Ni₃S₂@CNTs and thereexists a strong bonding between the CNT roots and Ni—F surface as well.The linear and tapering structure of Ni₃S₂@CNTs along with theirspherical roots and the catalyst particle enclosing the tips (FIGS. 6d-6 e ) match exactly with the morphology of Ni₃S₂@CNTs synthesizedusing nickel salt (FIGS. 3 d-3 f ), meaning that Ni₃S₂@CNTs follow thesame growth mechanism irrespective of the synthesis methods of Example 1and Example 2. Ni₃S₂ nanowires can be clearly distinguished at the coreof Ni₃S₂@CNTs as shown in FIG. 6 e.

Ni₃S₂@CNTs grown on Ni—F can be easily collected through a simple andquick process. An individual Ni—F substrate containing Ni₃S₂@CNTs wasimmersed in a clean beaker containing 2.5 ml of IPA. The beaker wasplaced in an ultrasonication bath for 5 minutes, and the substrate wasturned upside down after each minute. The ultrasonication process couldeasily disengage Ni₃S₂@CNTs from the Ni—F substrate, which was reflectedas a change in the color of transparent IPA into a turbid dispersion.Also, a simple magnetic purification process was used to collectNi₃S₂@CNTs with a high purity. After the collection of Ni₃S₂@CNTs, asmall piece of a clean stirring magnet was immersed in the beaker andstirred for 5 min. The magnet can easily attract metallic impuritiessuch as the tiny fragments of Ni—F falling off from the edges of Ni—Fduring ultrasonication. The process was repeated for several times untilall of the metallic Ni particles were removed from the dispersioncontaining Ni₃S₂@CNTs. FIG. 6 f shows the SEM image of a thin film ofpurified Ni₃S₂@CNTs spin-coated on a piece of clean Si wafer. To collecta significant mass (e.g., >50 mg) of pure powder samples (Ni₃S₂@CNTsclusters), the purified dispersion was transferred to another cleanbeaker and heated in a hot plate maintained at 150° C., and the wholeprocess was repeated using about 8-10 Ni—F substrates. The pure form ofNi₃S₂@CNTs thus collected can be plainly seen in FIGS. 7 a and 7 b . Inaddition, it also shows that the ultrasonication process can effectivelybreak the nanotubes and open them along their cross-section if needed.

Besides the simplicity of the one-step synthesis method, it is alsocost-effective as the same Ni—F substrate can undergo a recycle processto grow a new batch of Ni₃S₂@CNTs. FIGS. 8 a and 8 b show SEM images ofNi—F after collecting the Ni₃S₂@CNTs via ultrasonication for 5 minutes.It is apparent that most of the nanotubes were washed away during theultrasonication process without any significant damage to the substrate.After drying off, the same substrate was reloaded with Ni NPs catalysts,and the synthesis was repeated under the same conditions.

The SEM images of the Ni—F substrate after regrowth of Ni₃S₂@CNTs areshown in FIGS. 8 c and 8 d . The re-grown Ni₃S₂@CNTs as seen in FIGS. 8c and 8 d exhibit similar morphology to those in FIG. 6 d . Also, nogrowth of Ni₃S₂@CNTs was observed in the second run if the substrate wasnot dip-coated with Ni catalyst particles. This confirms that Ni—F canassist the growth of Ni₃S₂@CNTs, but the actual catalyst particlesresponsible for the growth of Ni₃S₂@CNTs were Ni NPs. Also, the catalystparticles get completely exhausted during the synthesis process suchthat there is no excess of unreacted catalyst particles available in thesubstrate for the further growth of Ni₃S₂@CNTs.

EXAMPLE 3

Microstructure of Ni₃S₂@CNTs

FIGS. 9 a-9 c show transmission electron microscope (TEM) images ofNi₃S₂@CNTs synthesized using Ni NPs. It can be seen in FIG. 9 a that thesynthesis method is highly reliable to produce CNTs filled with Ni₃S₂nanowires several micrometers long. Prior to the TEM studies, Ni₃S₂@CNTswere collected from the Ni—F using ultrasonication, which causedfragmentation of some of the Ni₃S₂@CNTs, but the majority of Ni₃S₂@CNTsremain intact. TEM examination showed that all CNTs are completely andcontinuously filled with Ni₃S₂ nanowires. FIG. 9 b shows the lowerresolution TEM image of an individual Ni₃S₂@CNT with a spherical rootenclosed by several layers of carbon shells. Also, it can be seen in theinset of FIG. 9 b that the Ni₃S₂@CNTs have their tips closed with thecatalyst particles.

FIG. 9 c shows the high resolution TEM image of a Ni₃S₂@CNT having ananowire diameter of about 36 nm and carbon shell thickness of about 30nm in each side. However, the thickness of nanotubes and nanowires mayvary from one Ni₃S₂@CNT to another. A continuum of lattice planes can beseen on both sides of the CNT/Ni₃S₂ interface revealing a completefilling in the radial direction too. As shown in the inset (on the rightof FIG. 9 c ), the interlayer spacing of the outer sheath is 0.34 nm,which matches with the (002) plane of graphitic carbon whereas theinterlayer spacing of 0.28 nm of the filler core matches with thed-spacing of the (110) plane of rhombohedral Ni₃S₂ (heazlewoodite phase(a=5.745 Å° and c=7.135 Å°; space group R32 (155); ICDD reference cardNo: 00-044-1418)). The selected area electron diffraction (SAED) patternobtained at the Ni₃S₂@CNT core shown in the inset on the left revealsthat the filler Ni₃S₂ nanowire is single-crystalline. The spots could beindexed as (011), (110), and (10-1) planes of rhombohedral Ni₃S₂ alongthe [1-11] zone axis. FIG. 9 d shows the drift corrected scanning andtransmission electron microscope (STEM) image taken at the middlesection of a particular Ni₃S₂@CNT about 350 nm thick along with ananowire diameter of about 130 nm whereas the inset shows the high-angleannular dark-field imaging (HAADF) STEM image of the same Ni₃S₂@CNT. Thecomposition of the nanohybrid was examined using energy dispersive X-rayspectroscopy (EDX), and the elemental mapping of C—K, Ni—K, Ni—L, andS—K are shown in FIGS. 9 e-9 h , respectively. The corresponding EDSspectra in FIG. 9 i plainly shows three peaks corresponding to Ni (K_(α)at 7.470 keV, K_(β) at 8.262 keV and L_(α) at 0.851 keV) and a majorK_(α) peak corresponding to Sat 2.306 keV, whereas the peakscorresponding to Cu and C are from the TEM copper grid and the CNTwalls, respectively. These results indicate that the synthesizedmaterials are Ni₃S₂@CNTs. To further confirm the structure of thesynthesized materials, X-ray diffraction and Raman spectroscopy wereemployed to examine the materials. In addition, the UV-Vis absorptionproperty of the materials was studied.

EXAMPLE 4

XRD and Raman Characterization of Ni₃S₂@CNTs

The crystal structure and phase composition of the Ni₃S₂ filler wasfurther confirmed using XRD measurements on the powder sample collectedfrom the Ni—F. The diffraction patterns marked with diamond symbols inFIG. 10 a can be indexed as (101), (110), (003), (021), (113), (211),(104), (122), (131), (214), and (401) planes of rhombohedral Ni₃S₂ (ICDDreference card No: 00-044-1418), which correspond to the 2θ values of21.75°, 31.10°, 37.77°, 38.27°, 49.73°, 50.12°, 54.61°, 55.16°, 69.27°,73.04°, and 77.89° respectively. The (002) peak for the graphite can beclearly observed at 26.36° whereas the three peaks (111), (200), and(220) marked with star symbols represent the diffraction patterns of Nicorresponding to the 20 values of 44.50°, 51.84°, and 76.36°respectively. The presence of the Ni peak in the XRD profile is probablyfrom the residuals of Ni—F in the sample.

FIG. 10 b shows the Raman spectra of empty multi-walled CNTs andNi₃S₂@CNTs in a frequency range of 120-2750 cm⁻¹. The position of D(1348 cm⁻¹), G (1597 cm⁻), and G′ (2683 cm⁻¹) bands for both samples arein good agreement with literature data for MWCNTs and other carbonaceousmaterials (see, e.g., DiLeo et al., Purity assessment of multiwalledcarbon nanotubes by Raman spectroscopy, Journal of Applied Physics, 101(2007) 064307, and Dorina et al., Raman Spectroscopy of CarbonaceousMaterials: A Concise Review, Spectroscopy, 26 (2011) 42-47; both ofwhich are hereby incorporated herien by reference in their entireties).G band features from CNTs are related to the E_(gg) mode of graphite,which arises due to the vibrations of sp² bonded C atoms in atwo-dimensional hexagonal lattice, whereas D band corresponds to thevibrations of sp³ carbon containing impurities, broken sp² bonding inside walls, lattice distortion (symmetry breaking defects) in hexagonalgraphitic layers as well as the doping effects. A very low I_(D)/I_(G)ratio of 0.26 confirms that CNT shells of the Ni₃S₂@CNTs synthesized bythe methods disclosed herein are very well graphitized and of ultra highcrystallinity. No related art methods have achieved an I_(D)/I_(G) ratiothis low for filled multi-walled CNTs. Unlike empty CNTs, Ni₃S₂@CNTsshow different Raman characteristics at lower frequencies due to thepresence of Ni₃S₂ nanowire at the interior of CNTs. The Raman bands at183, 195, and 216 cm⁻¹ shown in the inset of FIG. 10 b match well withthe known values of the Raman bands corresponding to the Raman activephonon modes of Ni₃S₂. The band observed at 152 cm⁻¹ is due to the straylight whereas the absence of other three bands of Ni₃S₂ at around 302,320, and 348 cm⁻¹ can be due to their dependence on the laser excitationwavelength or any sort of nanotube-filler interaction. The XRD and Ramanresults further confirmed the structure and chemical compostion ofNi₃S₂@CNTs.

EXAMPLE 5

UV-Vis Spectroscopy of Ni₃S₂@CNTs

The optical absorption properties of Ni₃S₂@CNTs were studied usingUV-Vis spectroscopy, and the results are shown in FIGS. 11 a and 11 b .In the experiment, empty CNTs and Ni₃S₂@CNTs were dispersed in ethanolto make samples with four different concentrations of 12.5, 25, 50, and100 mg⁻¹. The dispersions were kept in an ultrasonication bath for 30mins to make them uniform and the UV-Vis spectra were recordedimmediately. A broad absorption peak at 276 nm along with a gradualdecrease with increasing wavelength for empty CNTs (FIG. 11 a ) is inwell agreement with the UV-Vis characteristics of empty multi-walledCNTs reported. Such absorption feature arises due to the collectiveoscillation of free electron gas (it-plasmons) in the conduction band ofCNTs. However, Ni₃S₂@CNTs showed different UV-Vis characteristics due tothe interactive effects between the CNT shells and Ni₃ S₂ filler. FIG.11 b shows that the broad peak due to the π-plasmons resonance hasvanished, and the absorption is almost independent on the wavelength ofincident light. The reason behind this situation can be explained by acharge transfer process that can occur from the CNT shells to the Ni₃ S₂filler.

A negative shift can occur in the binding energy of S 2P₃₁₂ spectrum ofNi₃ S₂ after covering it with graphene layer along with the existence ofC—S bond in the C is spectrum, revealing the electron transfer fromgraphene to Ni₃S₂. In addition to the unique absorption properties ofNi₃S₂@CNTs, a linear dependence of absorption on the concentration ofboth empty and filled CNTs was also found (shown in FIG. 12 ), which isin compliance with Beer-Lambert law. A fairly constant value ofabsorption at all wavelengths for each concentration of Ni₃S₂@CNTssuggests that the molar absorption coefficient (c) of Ni₃S₂@CNTs isindependent of the energy of radiation in the UV-Vis region.

EXAMPLE 6

Synthesis of Cobalt Sulfide Nanowires Filled Carbon Nanotubes(Co₉S₈@CNTs) using cobalt metal nanoparticles

Commercial cobalt nanoparticles (Co, 99.8%, 28 nm) were used as catalystto synthesize Co₉S₈@CNTs on silicon substrates. The synthesis method wasgenerally the same as that in Example 2.

The SEM images of the synthesized Co₉S₈@CNTs are shown in FIGS. 13 a-13d . The morphology of the cobalt sulfide filled CNTs (e.g., sphericalroots, linear and tapered structure, and catalyst particle attached atthe tip) as seen in FIGS. 13 a-13 d indicate that they have a similarstructure as those of Ni₃S₂@CNTs. The CNTs shown in FIGS. 13 a-13 d arecompletely and continuously filled with cobalt sulfide nanowires severalmicrometers long. The presence of continuous cobalt sulfide nanowires atthe CNT cores can be plainly observed in FIGS. 13 c and 13 d . Thismethod can also be applicable to synthesize cobalt sulfide nanowiresfilled CNTs on metal foam substrates. Further examination showed thatcobalt sulfide filler is Co₉S₈ with cubic crystal structure, and that iswhy the cobalt sulfide nanowires filled carbon nanotubes are denoted asCo₉S₈@CNTs.

EXAMPLE 7

Synthesis of Co₉S₈@CNTs using cobalt salt

Cobalt salt Co(NO₃)₂.6H₂O was used as precursor to synthesize Co₉S₈@CNTson silicon substrates. The synthesis method was generally the same asthat in Example 1. The results were similar to those seen in FIGS. 13a-13 d for Example 6.

EXAMPLE 8

Synthesis of Iron Sulfide Nanowires Filled Carbon Nanotubes(Fe_(x)S_(y)@CNTs) Using Iron Metal Nanoparticles

Commercial iron nanoparticles (Fe, 99.5+%, 35-45 nm) were used ascatalyst to synthesize Fe_(x)S_(y)@CNTs on nickel foam. The synthesismethod was generally the same as that in Example 2.

The SEM images of the synthesized Fe_(x)S_(y)@CNTs are shown in FIGS. 14a-14 d . The morphology of iron sulfide filled CNTs grown at the edgesof Ni foam layers (FIGS. 14 a and 14 b ) is slightly different than thatof nickel or cobalt sulfide filled CNTs due to their partial filling. Itis evident from FIG. 14 b that CNTs are partially filled with ironsulfide nanowires about 10 μm length along their tips. However, in someCNTs lying at the top surface of the substrate, it was observed thattheir cores were continuously filled with longer iron sulfide nanowiresas shown in FIG. 14 c . Further, a clear distinction of the filledcore-shell structure of iron sulfide filled CNTs can be observed in thehigh magnification SEM image shown in FIG. 14 d . This method can alsobe used to synthesize iron sulfide filled CNTs on two-dimensionalsubstrates including but not limited to silicon or quartz. The fillerinside the CNTs was confirmed as iron sulfide, and that is why thesefilled CNTs are denoted as Fe_(x)S_(y)@CNTs.

EXAMPLE 9

Synthesis of Fe_(x)S_(y)@CNTs Using Iron Salt

Iron salt Fe(NO₃)₃.9H₂O was used as precursor to synthesizeFe_(x)S_(y)@CNTs on silicon substrates. The synthesis method wasgenerally the same as that in Example 1. The results were similar tothose seen in FIGS. 14 a-14 d for Example 8.

EXAMPLE 10

Synthesis of Metal Sulfide Nanowire Filled Single-Walled CNTs

The narrow diameter possessed by single-walled carbon nanotubes (SWCNTs)not only causes their aspect ratio to be much higher than that ofmulti-walled carbon nanotubes (MWCNTs) but also forces the encapsulatedmaterials to adopt a true one-dimensional morphology. New properties,phases, structures and behaviors of the filled CNTs can emerge as theircavity diameter becomes very small such as in SWCNTs. The in situfilling of SWCNTs with metal sulfides (e.g., nickel sulfide, cobaltsulfide, and iron sulfide) can be achieved using a similar processdescribed in Examples 1, 2, and 6-9 along with some modification in theprecursor and the experimental set up. A floating catalyst chemicalvapor deposition (FCCVD) technique can be employed to fill the SWCNTcores with atomically thin nanowires of metal sulfides. Metallocene(e.g., nickelocene, cobaltocene, or ferrocene) can be used as thecatalyst precursor, which can release tiny metal catalyst nanoparticlesduring their sublimation. Importantly, a sulfur containing chemical suchas thiophene (C₄H₄S) can be used as both the carbon source and sulfursource to guarantee both the growth of the SWCNTs and their in situfilling by metal sulfides nanowires. The in situ FCCVD can allow largescale production of metal sulfide filled SWCNTs. FIG. 15 shows aschematic representation of the in situ FCCVD method that can be used tosynthesize metal sulfide filled SWCNTs.

A certain amount (e.g., 10-20 mg) of metallocene (e.g., nickelocene,cobaltocene, or ferrocene) powder (M) can be placed on a quartz boatinside a quartz tube. Ar gas can be passed into the chamber at a flowrate of, e.g., 80-200 sccm for 10 mins by closing valves B and C so asto create an inert atmosphere inside the reaction tube. With the Arcontinuing to flow at the same rate, the temperature of the furnace (#2)can be raised to and maintained at 100-200° C. so that the sublimationof metallocene powder can proceed. Valve B can then be opened to pass H₂gas at a flow rate of, e.g., 80-200 sccm, which will reduce thevaporized metallocene to form metal atoms and subsequently agglomerateinto metal nanoparticles. The temperature of the second furnace (#3) canbe maintained at 850-1200° C., and the catalyst vapor can be fed intothe synthesis zone of the Furnace (#3) by increasing the Ar flow ratesignificantly (e.g., to 1500-1900 sccm). During a typical synthesis timeof 10-30 mins, the direction of H₂ gas flow can be switched from valve Bto C so that it bubbles through thiophene and passes into the tube.Thus, the thiophene vapor will be carried into the reaction chamber, anda proper sulfidation reaction can occur resulting in the simultaneousgrowth and filling of SWCNTs with metal sulfide nanowires. The finalproduct is of a macroscale of SWCNTs filled with metal sulfide nanowireseither in form of a web-like thin material or a thick nonwoven materialformed at the rear side of the quartz tube.

It should be understood that the examples and embodiments describedherein are for illustrative purposes only and that various modificationsor changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication.

All patents, patent applications, provisional applications, andpublications referred to or cited herein are incorporated by referencein their entirety, including all figures and tables, to the extent theyare not inconsistent with the explicit teachings of this specification.

What is claimed is:
 1. A method of synthesizing filled carbon nanotubes(CNTs), the method comprising: providing a substrate in a reactionchamber; disposing a catalyst comprising a metal on the substrate;providing a sulfur-containing organic compound to the reaction chamber;and performing a chemical vapor deposition technique to simultaneouslygrow CNTs and fill the CNTs with a metal sulfide comprising the metal,thereby synthesizing the filled CNTs filled with the metal sulfide, theCNTs having an average filling ratio of the metal sulfide of at least0.90.
 2. The method according to claim 1, the metal being nickel,cobalt, or iron.
 3. The method according to claim 1, the metal beingnickel and the metal sulfide being Ni₃S₂.
 4. The method according toclaim 1, the sulfur-containing organic compound being thiophene ordimethyl sulfide.
 5. The method according to claim 1, the providing ofthe sulfur-containing organic compound to the reaction chambercomprising: providing a supply of the sulfur-containing organic compoundoutside the reaction chamber; and sending a gas through the supply ofthe sulfur-containing organic compound and into the reaction chambersuch that the sulfur-containing organic compound is bubbled into thereaction chamber with the gas.
 6. The method according to claim 1, thecatalyst being a salt of the metal, and the method further comprising:before providing the sulfur-containing organic compound to the reactionchamber, heating the reaction chamber to a first predeterminedtemperature for a first predetermined amount of time to oxidize thecatalyst; after oxidizing the catalyst, flowing an inert gas through thereaction chamber and passing hydrogen gas through the reaction chamberto reduce the catalyst; and after reducing the catalyst, stopping theflowing of the hydrogen gas through the reaction chamber whilecontinuing to flow the inert gas through the reaction chamber whileheating the reaction chamber to a second predetermined temperature. 7.The method according to claim 6, the disposing of the catalyst on thesubstrate comprising: preparing a catalyst solution by mixing a powderof the salt of the metal in a solvent; providing the catalyst solutionto the substrate; and performing a two-stage spin coating to form auniform coating of the catalyst on a surface of the substrate.
 8. Themethod according to claim 7, the catalyst solution having aconcentration of the salt of the metal in a range of 0.02 molar (M) to0.06 M.
 9. The method according to claim 7, further comprising, beforedisposing the catalyst on the substrate, cleaning the substrate viaultrasonication by performing three successive ultrasonication processesusing deionized water, acetone, and isopropyl alcohol, respectively. 10.The method according to claim 1, the catalyst being nanoparticles of themetal, and the method further comprising: before disposing the catalyston the substrate, dispersing the nanoparticles in a solvent and puttingthe solvent in an ultrasonication bath to achieve a uniform suspensionof the nanoparticles in the solvent; dip coating at least a portion ofthe substrate by immersing the at least a portion of the substrate in acontainer having the suspension of the nanoparticles in the solvent andgently shaking the container; and transferring the at least a portion ofthe substrate to a heating apparatus and heating the at least a portionof the substrate to evaporate the solvent.
 11. The method according toclaim 10, the substrate being a foam comprising the metal, and themethod further comprising: before disposing the catalyst on thesubstrate, cleaning the substrate via ultrasonication using at least onesolvent.
 12. The method according to claim 11, the cleaning of thesubstrate via ultrasonication comprising performing three successiveultrasonication processes using deionized water, acetone, and isopropylalcohol, respectively.
 13. The method according to claim 1, furthercomprising: collecting the filled CNTs via ultrasonication; and using amagnet to purify the filled CNTs during the collecting, after thecollecting, or both.
 14. The method according to claim 13, furthercomprising, after collecting the filled CNTs, reusing the substrate tosynthesize further filled CNTs.
 15. A method of synthesizing filledcarbon nanotubes (CNTs), the method comprising: providing a substrate ina reaction chamber; disposing a catalyst comprising a metal on thesubstrate; providing a sulfur-containing organic compound to thereaction chamber; performing a chemical vapor deposition technique tosimultaneously grow multi-walled CNTs and fill the multi-walled CNTswith a metal sulfide comprising the metal, thereby synthesizing thefilled CNTs filled with the metal sulfide; collecting the filled CNTsvia ultrasonication; and using a magnet to purify the filled CNTs duringthe collecting, after the collecting, or both, the multi-walled CNTshaving an average filling ratio of the metal sulfide of at least 0.99,the multi-walled CNTs having an average length of at least 10 μm, themetal being nickel and the metal sulfide being Ni₃S₂, the providing ofthe sulfur-containing organic compound to the reaction chambercomprising: providing a supply of the sulfur-containing organic compoundoutside the reaction chamber; and sending a gas through the supply ofthe sulfur-containing organic compound and into the reaction chambersuch that the sulfur-containing organic compound is bubbled into thereaction chamber with the gas, the sulfur-containing organic compoundbeing thiophene, the method further comprising, before disposing thecatalyst on the substrate, cleaning the substrate via ultrasonicationusing at least one solvent, and the reaction chamber comprising a tubeof a first material with a boat of the first material disposed therein,the boat comprising a surface to which the substrate is provided. 16.The method according to claim 15, the catalyst being a nickel salt, themethod further comprising: before providing the sulfur-containingorganic compound to the reaction chamber, heating the reaction chamberto a first predetermined temperature for a first predetermined amount oftime to oxidize the catalyst; after oxidizing the catalyst, flowing aninert gas through the reaction chamber and passing hydrogen gas throughthe reaction chamber to reduce the catalyst; and after reducing thecatalyst, stopping the flowing of the hydrogen gas through the reactionchamber while continuing to flow the inert gas through the reactionchamber while heating the reaction chamber to a second predeterminedtemperature, the disposing of the catalyst on the substrate comprising:preparing a catalyst solution by mixing a powder of the nickel salt in asolvent; providing the catalyst solution to the substrate; andperforming a two-stage spin coating to form a uniform coating of thecatalyst on a surface of the substrate, the catalyst solution having aconcentration of the nickel salt in a range of 0.02 molar (M) to 0.06 M,and the cleaning of the substrate via ultrasonication comprisingperforming three successive ultrasonication processes using deionizedwater, acetone, and isopropyl alcohol, respectively.
 17. The methodaccording to claim 15, the catalyst being nickel nanoparticles, themethod further comprising: before disposing the catalyst on thesubstrate, dispersing the nickel nanoparticles in a solvent and puttingthe solvent in an ultrasonication bath to achieve a uniform suspensionof the nickel nanoparticles in the solvent; dip coating at least aportion of the substrate by immersing the at least a portion of thesubstrate in a container having the suspension of the nickelnanoparticles in the solvent and gently shaking the container; andtransferring the at least a portion of the substrate to a heatingapparatus and heating the at least a portion of the substrate toevaporate the solvent, the substrate being nickel foam, and the cleaningof the substrate via ultrasonication comprising performing threesuccessive ultrasonication processes using deionized water, acetone, andisopropyl alcohol, respectively.